Newt Gingrich has said he will quit the race for the Republican presidential nomination and will throw his political weight behind Mitt Romney for the presidency in what he called the "most important election in our lifetime".

Speaking in a web video to thank supporters, the former speaker of the House of Representatives did not endorse any other candidate, and made no mention of Romney by name. But he did say he will be campaigning to prevent Barack Obama's re-election in November, the prospect of which he called a "genuine disaster". He said he would officially suspend his campaign on Wednesday.

"The issues are the same," he said. "We're still faced with a tremendous crisis of our country's future. A re-election of Barack Obama will be a genuine disaster. And all of us have an obligation, I think, to do all we can to defeat Barack Obama."

Gingrich's withdrawal from the race leaves only Ron Paul as a distant challenger to Romney for the Republican nomination.

Gingrich thanked his supporters for "a terrific campaign", which peaked with his thumping victory over Romney in South Carolina as southern Republican voters turned away from the former Massachusetts governor – in part over suspicion of his previous support for abortion, gun control and mandatory healthcare insurance. Others were suspicious of his Mormon religion which some regard as a cult not a part of Christianity.

But Gingrich's campaign quickly imploded after that when he failed to win Florida a week later and, besides a victory in his home state of Georgia, then faced a string of losses including in five north-eastern states last week.

Gingrich's bid for the presidency imploded in part because of erratic performances in televised debates, his inflammatory rhetoric, such as saying that as president he would not be bound by supreme court rulings he disagreed with, and outlandish ideas, including a proposal to establish a colony on the moon which was met with scorn.

Romney hit Gingrich with a slew of campaign adverts reminding voters that the former House speaker was sanctioned by Congress for ethical violations and about his work lobbying for government-sponsored mortgage corporations he had also condemned in speeches.

Some conservatives were also put off by his serial adultery and three marriages, although Gingrich attempted to portray himself as a reformed character after converting to Roman Catholicism.

Other Republican voters feared he stood a far weaker chance than Romney of beating Obama.

Gingrich told supporters in North Carolina last week that he would be "transitioning" his campaign, which was widely interpreted as meaning he intended to quit the race.

His campaign is reported to be millions of dollars in debt. By suspending and not formally abandoning the campaign he can continue to raise funds to clear the debt.